Known electric double layer capacitors of the type mentioned above are comprised of, as shown in FIG. 1, polarizable electrodes 1 which are obtained by press molding of active carbon particles, by applying a mixture of active carbon particles and an appropriate binder onto a collector metal, or by forming a spray coating layer of aluminium on active carbon fibers. The polarizable electrodes 1 are accomodated in a stainless steel case 2 and are facing each other through an electrolytic solution and a separator 3. The metallic case 2 is sealed through a gasket 4 at a peripheral opening thereof.
Another type of known capacitor now in use is shown in FIG. 2 in which a non-polarizable electrode 5 is used as one electrode.
In these known arrangements, solvents used for the electrolytic solution are propylene carbonate, r-butyrolactone, N,N-dimethylformaldehyde, and acetonitrile. In the case of polarization at the anode, the stainless steel in the electrolytic solution cannot make a complete passive state but dissolves in the solution. The potential at which the current starts to run owing to the dissolution is 2.3-2.4 volts which is determined depending on the decomposition potential of the solvent at the cathode. This is lower than the potential of oxidation of active carbon or the potential of decomposition of the electrolyte in the electrolytic solution using the organic solvents. Accordingly, when the stainless steel case 2 is used as a current collector, the potential at the anode is restricted by the potential of dissolution of the stainless steel, so that a potential of 3 V which is in an electrochemically stable potential region determined by the polarizable electrode 1 and the electrolytic solution could not be effectively utilized.
For instance, when an excess voltage over a voltage at which a leakage current starts to increase is applied, large amounts of iron, nickel and the like are detected in the electrode at the side of the anode, from which it has been confirmed that the stainless steel is dissolved and iron ions are moved from the anode toward the cathode.
As will be seen from the above, when stainless steel is used as the metallic case 2, it is difficult to effectively utilize a potential of 3 V which is in an electrochemically stable region determined by the active carbon polarizable electrode 1 and the electrolytic solution. In order to obtain an electric double layer capacitor of a breakdown voltage which enables one to use 3 V, it is necessary to use a material which allows passage of a reactive current at a potential equal to or larger than the active carbon of the polarizable electrode 1 and which has sufficient strength when the anode is subjected to polarization in a solvent used.
One such a material may be titanium which forms a passive state in electrolytic solutions. As shown in FIG. 3, however, the breakdown voltage becomes higher than in the case using stainless steel. With electrolytic solutions using propylene carbonate and tetraethylammonium perchlorate, the region where the reactive current flows increases by about 0.8 V. However, the internal resistance increases, so that when the voltage drop becomes large in the case where the electric double layer capacitor is used, there is an attendant problem that such a capacitor cannot be in use. The present invention is contemplated to solve the above problem and has for its object the provision of an electric double layer capacitor which has a high breakdown voltage over 3 V.